Chronological skin aging is a universal and inevitable process characterized by physiological alterations in keratinocyte activities and epidermal functions as well as dermal changes. Epidermal dysfunction and abnormal keratinocyte activities in aged skin often lead to severe clinical consequences such as delayed wound healing, various skin diseases and morbidity in elderly Veterans patients. At the present time, the cellular and molecular mechanisms causing epidermal dysfunction and skin aging are not well understood. However, recent studies reveal abnormal extracellular matrix hyaluronan (HA) metabolism may be involved in the changes associated with keratinocyte activities, permeability barrier homeostasis and wound healing during skin aging. In this proposal, we plan FIRST to test the hypothesis that matrix HA and its catabolic products (e.g., large and small HA) selectively activate CD44 (HA receptor)-mediated RhoGTPase (RhoA/Rac) signaling. Specifically, we will assess the extent to which small HA promotes CD44-dependent RhoA activation, and ROK-regulated transcriptional activation and migration/proliferation (together with pro- inflammatory growth factor/cytokine production) in mouse and human keratinocytes. In parallel, we will also determine whether large HA activates CD44-specific Rac signaling and PKN?-mediated differentiation (lipid synthesis/lamellar body formation) and increases the survival/life span of mouse and human keratinocytes. Complementary biochemical and immunohistochemical techniques will be used to elucidate HA-CD44 interactions with various signaling molecules (e.g., RhoA/ROK-regulated Raf-1ERK and NIK-NFkB as well as Rac1-PKN?-regulated p38MAPK-AP1 & SIRT1) that promote keratinocyte activities, keratinocyte-fibroblast interactions and skin functions, including permeability barrier homeostasis and wound healing. Our SECOND hypothesis states that aberrant HA metabolism and CD44 signaling contribute to the decline in RhoA- ROK/Rac-PKN? signaling accounting for abnormal keratinocyte function and keratinocyte-fibroblast interactions in aging skin which result in multiple, adverse clinical consequences. Specifically, corrective strategies will be explored using aged mouse models. Unique HA-based therapeutic strategies will be tested and molecular mechanism leading to the aged phenotype will be identified. The new knowledge obtained from our proposed studies will allow us to delineate specific HA (small vs. large)-mediated signaling events (through the CD44 receptor) and subsequent keratinocyte activities and keratinocyte-fibroblast interactions. In addition, it will provide important new information tht can lead to improved therapeutic options for Veterans patients suffering aging-related skin diseases, such as chronic non-healing wounds, epidermal thinning (atrophy) & barrier dysfunction. As such, this proposed project clearly has important clinical potential for the development of novel treatment approaches and improved care of Veterans patients with aged-related delay in skin wound repair and other skin diseases. Therefore, this proposal is directly relevant to both the rehabilitation program goals and the VA patient care mission.